An illusion according to the dictionary is "something that decieves by producing a false or misleading impression of reality." Tom's life fits this description accurately, particularly in his understanding of himself, and his marriage.

In the early chapters, Tom likes to think he's an intellectual, but the truth is he has little understanding of that which he reads. In this way, his life is an illusion to him.

Perhaps the greatest illusion for Tom is his understanding of his marriage with Daisy. Although he continually in an affair with Myrtle, he expects that Daisy is completely faithful and whole-heartedly in love with him. By chapter 7, Daisy is blatantly revealing her affair with Gatsby. As Tom figures out what is going on, he realizes his entire marriage is a sham. This is an illusion uncovered for him.